British Pat. No. 792,598 to Ranby and corresponding J. Electrochemical Soc., Vol. 104, No. 10 (October 1957) pp. 612-615, article entitled "Cerium-Activated Halophosphate Phosphors" by S. T. Henderson and P. W. Ranby disclose a luminescent material comprising a halophosphate of calcium and/or strontium, having an apatite structure activated with cerium or with cerium and manganese, and the phosphor materials of optimum brightness being formulated as: EQU Sr.sub.4.57 Ce.sub.0.2 Mn.sub.0.25 (PO.sub.4).sub.3.05 F.sub.5.2.
It has been found that the optimum brightness is not at the Henderson and Ranby formulation.